This invention relates to improvements in radar apparatus using microwave frequency components, and in particular to a radar circuit, which incorporates temperature compensation for microwave components.
It is known to use radar in the measurement of distance. In one application radiation in the microwave region of the spectrum is emitted from a source towards a target. The target will reflect some of this radiation back towards a detector, which is sensitive to radiation in the microwave region of the spectrum. An electronic circuit, which may include a digital signal processor, is then used to make a comparison between the emitted signal and the measured, detected, signal from which the range of the target from the source/detector can be estimated.
It is known to provide a frequency shift key scheme to determine the range from the signals. In a vehicle application, where the “targets” are other vehicles, which precede the vehicle to which the radar apparatus is fitted, observations are made of the vehicles as they move along the road. The motion of these vehicles is tracked over time to provide a consistent view of their time varying distribution on the road ahead. To achieve this the radar estimates each vehicle's range and relative velocity from the microwave signals.
More precisely, distance can be measured from the phase difference that is seen in the returned signal when the emitted signal is modulated by a small step change in frequency as given by:
  d  =            c      ⁢                          ⁢              ϕ        s                    4      ⁢      π      ⁢                          ⁢              f        s            Where c is the speed of light, φ, is the relative phase of the Doppler signals returned by two frequencies separated by fs, the frequency step size. The accuracy of the measurement is related to the frequency step size.
Similarly, the relative velocity can be measured by looking at the Doppler shift between the emitted signal and the received signal as given by the following expression:
  v  =            cf      o              2      ⁢              f        D            where c is the speed of light, fo is the carrier frequency and fD is the Doppler shift.
As the carrier frequency is regulated by a highly accurate resonator in the microwave components, it is very stable and the velocity measurements are highly accurate.
A problem with such apparatus is that variations in the temperature of the microwave frequency generator cause errors in the estimation of the target vehicles distance. This has in the past been solved by providing a heater to regulate the temperature of the frequency generator and in particular to raise its temperature to an ideal or target region in which the temperature co-efficients of the components are linear to allow open loop corrections to be applied to the modulation. A temperature sensor is used to provide the information needed to drive these control circuits.
We have realised that the reliance upon a temperature sensor in a scheme of this type can result in unsatisfactory performance after extended use in harsh environments due to significant ageing effects on the temperature sensor.